Thermoluminescence study of pellets prepared using NaCl from Khewra Salt Mines in Pakistan.

Abstract

In this study, the thermoluminescence characteristics of naturally occurring salt (NaCl) were assessed for the development of a radiation dosimeter. For this purpose, mined crystalline samples of salt were procured directly from Khewra salt mines in Pakistan. The samples were hand crushed, sieved, and compressed to pellets comparable in size to standard TLD chips, and irradiated to gamma radiation doses in the range of 5mGy and 5000mGy. Thermoluminescence (TL) response showed three main peaks in the glow curve around 115-130°C, 150-170°C, and 220-240°C. A linear TL response was observed for the dose range of 5-100mGy. The TL response became supra-linear for the dose ranges of 100-1000mGy and 1000-5000mGy. The Tm-Tstop method was applied to identify the overlapping peaks of the glow curve. Computerized glow curve deconvolution (CGCD) was then employed for the characterization of electron trap parameters such as frequency factor (s), activation energy (E), and the kinetic order (b), using General Order (GO) kinetics. The figure-of-merit (FOM) was found to be 1.08%, 0.94%, 0.77%, and 0.75%, at 500mGy, 1Gy, 2Gy, and 5Gy, respectively. The TL intensity faded by 20% within the first 24h after irradiation and finally stabilized after two weeks. In addition, structural, morphological, and elemental analyses, were also performed using various analytical techniques. X-ray diffraction (XRD) showed that the salt crystallizes in a face-centered cubic structure. Scanning electron microscope (SEM) micrographs indicated that the crystallites are closely packed and cubic-shaped with non-uniform size, and mostly found in the agglomerated form. Similarly, the elemental analysis confirmed the presence of impurities such as Mg, Sr, S, K, O, and Ca, in the samples. The present study concludes that the pellets made from salt samples from Khewra mines have a potential for use as radiation dosimeters.